Change repo's 'crystal-language' topic to 'crystal'

[matiasgarciaisaia]

The Crystal community has somewhat decided to use crystal as the language's topic.

To avoid fragmentation in the community, please change the crystal-language topic, that only has ~30 repositories at the moment, to crystal.

Any contributor can change it from the repo's homepage.

See crystal-lang/crystal#4241 for more context.

Thank you very much :)

[werner]

Thanks!